得到一些奇怪的错误

Question

我正在我的高中学习计算机科学，我们正在用Java编程，对于我的最终项目，我正在为天文学制作一个自动数学计算器。我正在尝试使用方程列表创建一个下拉框，我已经编写了下拉框和动作侦听器以及常规GUI，我收到错误＆＃34;类型mainScreen必须实现继承的抽象方法。我也遇到了很多其他错误，但这是主要错误。 java.awt.event.ActionListener.actionPerformed（java.awt.event.ActionEvent中）＆＃34 ;.抽象课我不是最好的。由于我还不熟悉Java编码，所以我不是最好的。这是我的代码：

import javax.swing.JFrame;
import java.awt.*;
import java.awt.event.*;

public class mainScreen extends JFrame implements ActionListener
{
 private static final String[] equations = {"Distance Mod", "Luminosity", "Mass of Binary"};
 JComboBox equationList = new JComboBox(equations);
 Jlabel whatEquation = new JLabel();

 public static void main(String[] args)
  {
    mainScreen equat = new mainScreen();
    centerFrame(equat);
    equat.setVisable(true);

  }
  public mainScreen()
  {
   setLayout(new Flowlayout());
   setSize(900, 800);
   setTital("Astronomy Equations");
   setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

   equationList.setSelectedIndex(1);
   equationList.addActionListener(this);
   add(equationList);
   add(whatEquation);
  }
  public void actionPreformed(ActionEvent e)
  {
    if(e.getSource() == equationList)
    {
      JcomboBox cb = (JComboBox)e.getSource());
      String msg = (String)cb.getSelectedItems();
      switch(msg)
      {
        case "Distance Mod" : whatEquation.setText("Please put in the known info and put a '0' if data is not known.");
            break;
        case "Luminosity" : whatEquation.setText("Please put in the known info and put a '0' if data is not known.");
            break;
        case "Mass of Binary" : whatEquation.setText("Please put in the known info and put a '0' if data is not known.");
            break;
        default : whatEquation.setText("Whoops. an error has occured please try again. :)");
      }
    }
  }
}